scott: TOXODONTIA OF THE SANTA CRUZ BEDS. 
293 
hyoid arch, which is ankylosed with the anterior end of the bulla in the 
adult skull. 
While very few of the indigenous South American ungulates were 
horned, all the known genera which display any indications of horns are 
referable to the Toxodonta. In the Pyrotherium Beds the Leontiniidae, 
which are usually assigned to the Entelonychia, seem to have had a nasal 
horn, and in the Santa Cruz one species of Nesodon (N cornutus ) and 
nearly or quite all the species of Adinotherium apparently had a very 
small frontal horn, which attains large proportions in Trigodon of the 
Monte Hermoso horizon. 
In the Entelonychia the skull is of the same type as in the Toxodonta 
and Typotheria, with several peculiar modifications. Except in the very 
ancient family of the Notostylopidae, the skull has less likeness to that of 
the rodents than in either of the other suborders and in the homalodon- 
totheres, which are the most specialized family of the Entelonychia, the 
skull has become very characteristic. The questionable element ( pars 
Serrialis), which I have regarded as belonging to the squamosal, is less 
extensively exposed on the occipital surface, and the mastoid portion of 
the periotic is not visible externally, so that there is no mastoid process. 
The external auditory meatus, which is an irregular hole, has a more 
inferior position than in either of the other groups, while the tympanic 
bulla is very large, with its principal diameter in the antero-posterior 
direction. The nasals are very short and the anterior nares present ob- 
liquely upward and forward ; the premaxillae are very small, far smaller 
than in the Toxodonta, though they are quite large in the more ancient 
and primitive genera, such as Thoniashuxleya and Asmodeus. The 
muzzle and chin are short and abruptly rounded. 
Vertebral column. — This varies considerably, as would of course be ex- 
pected from the very different stature and bulk of the three suborders, 
but there is, nevertheless, a general uniformity of structure. In all the 
neck is short, or of only moderate length, and the axis always has the 
conical, peg-like odontoid and large, hatchet-shaped neural spine. The 
trunk is long, with 20-22 vertebrae in the Toxodonta and Typotheria and 
in the larger genera of the former, such as Nesodon and Toxodon , the 
neural spines are very elongate in the anterior thoracic region and form a 
decided hump at the shoulders. The tail is stout and of moderate length 
in the Toxodonta, very long in the Santa Cruz Typotheria, except in 
